This invention relates to a vertical position welding method for butt-welding base metals, or steel plates, and to a welding apparatus for practicing the method.
An electroslag welding method and an electrogas welding method are known as automatic vertical position welding, and are popularly employed especially in the field of shipbuilding because these welding methods are suitable for welding large structures on the spot, and are advantageous in that, for instance, the welding operation can be readily achieved, and the weld is scarcely deformed.
However, the conventional vertical position welding methods have a common difficulty that metal plates to be welded must be heated at very high temperture, which leads to the deterioration of the physical properties, such as impact values, of the portions of the metal plates affected by the heat.
In other words, in the conventional electro-slag or electro-gas welding method, the welding is conducted by maintaining the relationships between the melting of the metal plates and the melting of the welding wire suitable by feeding electric current between the welding wire and the metal plates (referred to as the work when applicable) and therefore a groove formed by the edges of the metal plates should have a relatively large width and the welding wire also should have a relatively large diameter so that the melting rate (that is, the welding rate) of the welding wire may not be greater than the melting rate of the work. This means that the welding must be conducted at a low speed so that the edges of the plates to be welded are sufficiently molten.
The fact that a welding groove has a large width is convenient for inserting a welding wire into the groove, but is undesirable due to the following reasons: It is obvious that a welding rate in the case of a wide groove is smaller than that in the case of a narrow groove, which leads to the application of excessive heat to the work. As a result, the quantity of metal molten from the work increases, and the physical properties of the weld in the work are deteriorated. Thus, it is impossible to employ the electro-slag or electro-gas welding method for welding materials such as thick plates, high-tension steels, and alloy steels which are greatly affected by heat.
Especially, the electro-slag welding, being a resistance welding utilizing molten slag, has a difficulty that the depth of penetration is insufficient at the start of welding, that is, lack of fusion is liable to be produced, which must be repaired by other welding method.
In order to solve the promblems described above, recently a narrow gap welding method has been proposed in which the width of a groove formed by metal plates to be welded is relatively small, and heat applied to the work is reduced by using a small current so as to prevent the deterioration of the physical properties of the work.
However, this narrow gap welding method also has difficulties. That is, in this method, it is necessary to oscillate a welding arc by an arc oscillating mechanism or to conduct a multiple pass welding, and its welding current cannot be increased and therefore the welding rate cannot be increased. That is, in this method, the reduction of heat applied to the work causes the lowering of its welding effeciency, and therefore the application of the narrow gap welding method is limited.
The above-described difficulties accompanying the conventional methods have been overcome by the provision of a vertical position welding method and a first welding apparatus practicing the method according to this invention. However, this apparatus should have been improved, because, in this apparatus, a plasma arc generating torch, or a non-consumable electrode torch is perpendicular to the plane of the work, and therefore an arc is liable to be produced between the torch and a molten metal pool in a groove, which is formed by the edges of metal plates to be welded, at the middle of the groove. As a result, it is impossible to sufficiently apply the energy of an plasma arc generated by the torch to the rear surface of the work, which leads to the production of cold laps on the rear surface of the work.
Referring back to the conventional vertical position welding apparatus, metal plates are molten by a plasma arc generated between a non-consumable electrode torch and the edges of the metal plates forming a groove therebetween, and metal molten from the groove forms a molten metal pool with an additive metal while the torch, that is, the plasma arc is moved upward. This molten metal pool is solidified to form beads. However, in the conventional apparatus, the position of the plasma arc is relatively far apart from the position of the molten metal, and undercuts are therefore liable to be produced. That is, it is necessary to provide a welding apparatus for practicing the vertical position welding method whose torch can be set to the groove as close as possible.
This object has been achieved by a welding apparatus provided according to another aspect of this invention, as will become apparent from the following detailed description. However, as the electrode of a non-consumable electrode torch is not electrically insulated from a front holding member or a front backing shoe in contact with the work, an arc tends to occur between the electrode and the front holding member, which leads to the insufficient application of the plasma arc to the work. Therefore this problem must be also solved.